Known in the art and extensively employed for electrochemical metallization of dielectrics are solutions having various compositions. Thus, use is made of solutions of surfactants for surface degreasing, i.e. imparting hydrophilic properties thereto; acid solutions of tin chloride to sensitize the surface; solutions of palladium chloride for activation of the surface, i.e. deposition, on the dielectric surface, of metallic palladium particles which catalyze the process of chemical copper plating; solutions for chemical copper-plating for application, onto the dielectric surface, of a thin (1 to 3 .mu.m) current-conducting layer; solutions of electrolytes for electroplating build-up of a metal layer of a required thickness (cf. USSR Inventor's Certificate No. 240061, published 21.03.1969, Bulletin No. 12, Cl. H 05 K 3/06).
Known in the art are also modifications of the abovementioned combination of solutions characterized, in particular, in that for the surface activation use is made of salts of noble metals (gold, silver, platinum and the like) and a current-conducting layer of other metals (nickel, cobalt, iron and the like) is chemically deposited onto the activated surface. Accordingly, the solutions for chemical metallization are also different and consist of a salt of a reducible metal (copper, nickel, cobalt, iron and the like), a reducing agent (sodium boron hydride, formaldehyde, hydrazine and the like) and a stabilizer--a solution of a complex-forming agent for the metal ions (citric acid, Seignette salt and the like), or a surface-active substance (dodecylaminacetate and the like) (cf. U.S. Pat. Nos. 3,515,649 of June 2, 1970; Int. Cl. C 23f 17/00; US Cl. 204-38; 3,553,085 published Jan. 5, 1971; Int. Cl. C23 d 5/60, US Cl. 204-30; 3,764,488 published Oct. 9, 1973, Int. Cl. B 44 d 1/09, US Cl. 240-381.3; 3,563,784 published Oct. 16, 1971, Int. Cl. C 23c 3/00, US Cl. 117-47).
Also known is a solution for electrochemical metallization of dielectrics employed for the surface activation before a chemical metallization and having the following composition: PdCl.sub.2 --0.5-1.0 g/l; H.sub.2 SO.sub.4 --40-200 ml/l; SnCl.sub.2 --30-50 g/l; HCl--10-50 ml/l (U.S. Pat. No. 3,650,913 published Mar. 21, 1972; Int.Cl. C 23b 50/60, US Cl. 204-30).
Also known is a combination of solutions for electrochemical metallization of through holes of multi-layer printed boards which differs from those specified hereinbefore by the additional use of a hydrochloric-acid solution of rhodium chloride prior to the activation of the surface of the hole walls which has the following composition: rhodium chloride--4.5-5 g/l; hydrochloric acid--250-200 mg/l (cf. USSR Inventor's Certificate No. 470940, published May 15, 1975, Bulletin No. 18, Int.Cl. H 05K (3/00).
All the above-mentioned solutions for electrochemical metallization contain economically inefficient and difficult-to-obtain substances such as salts of noble metals (palladium, gold, silver, platinum, rhodium), stabilizing agents such as citric acid and Seignette salt. The above-discussed prior art solutions for electrochemical metallization have low stability, thus necessitating frequent correction and replacement. These solutions adapted for electrochemical metallization do not ensure a required quality of application of chemical coatings and an adequate reproducibility thereof.
Also known in the art is a solution for electrochemical metallization of dielectrics which has the following composition, g/l: copper salt (as calculated for metallic copper)--35-40; potassium pyrophosphate--450-500; ammonia--3-6; citric acid--10-20 (cf. USSR Inventor's Certificate No. 159368 published 1963. Bulletin No. 24, Cl. C 23 B 5/18). This solution has but a weak activation power relative to dielectrics, wherefore it does not ensure a required quality of the current-conducting layer intended for electroplated building-up of a metal coating on the dielectric surface.
Known are different processes for electrochemical metallization of dielectrics which have found an extensive use in the art.
Thus, electrochemical metallization of dielectrics is effected in the following manner. The surface is degreased and washed with water. Then the surface is activated, prior to the deposition of the current-conducting layer, by treatment thereof with a solution of tin chloride, washing, followed by treatment with a solution of palladium chloride and washing. As a result of such activation, on the surface there are formed particles of metallic palladium which serve as initiators of a chemical copper-plating process. After the latter chemical copper-plating there is effected the formation of the current-conducting layer on the dielectric surface. After the subsequent washing, the coating of the required thickness is obtained by electroplating build-up of the metal. In this manner, the combination of successive operations constituting this process is as follows: degreasing, washing, activation, formation of a current-conducting layer (by chemical copperplating), washing, electroplating build-up of the coating (cf. USSR Inventor's Certificate No. 240061, published Mar. 21, 1969, Bulletin No. 12, Cl. H 05 K 3/06).
Another prior art process for electrochemical metallization of dielectrics differs form the above-described in that the surface activation is carried out with the use of a combined solution of tin chloride and palladium chloride which makes it possible to combine operations of sensitization and activation into one operation and avoid one washing operation (cf. U.S. Pat. No. 3,650,913 published Mar. 21, 1972, Int.Cl. C 23 B 5/60, US Class 205-30).
The above-mentioned processes for electrochemical metallization of dielectrics are complicated due to their multistage character, long duration of the process, necessity of preparation and frequent correction of unstable solutions of tin chloride, combined activation solutions, solutions of chemical metallization, necessity of utilization of wastes of precious metals of palladium and rhodium, increased labour-consumption and unsatisfactory reproducibility of chemical metallization processes. Furthermore, these processes contemplate the use of economically inefficient and difficult-to-obtain substances such as salts of palladium, gold, silver, platinum, rhodium, citric acid, Seignette salt and the like.
Also known is a process for electrochemical metallization employed for metallization of walls of through holes in multi-layered printed boards which, in addition to the operations of the above-discussed processes, has an operation of the surface treatment in a hydrochloric-acid solution of rhodium chloride after operations of degreasing and washing. The purpose of this operation is as follows. If metallization is carried out by this process, then in the treatment of hole walls in the solution of palladium chloride there occurs a contact formation of a film of palladium metal on end faces of copper contact plates. This palladium film during manufacture and use of a printed board effectively absorbs hydrogen while being transformed into palladium hydride--a brittle and non-conducting material--which results in breaking of the mechanical and electric contact and the board gets inoperative. The introduction of the operation of treatment of wall of holes in a solution of rhodium chloride makes it possible to deposit, on end faces of a copper foil, a protective film of metallic rhodium preventing the future formation of a film of palladium. (Cf. USSR Inventor's Certificate No. 470940 published May 15, 1975, Bulletin No. 18, Int.Cl. H 05 K 3/00).
This process, likewise those described hereinabove, is multi-staged, consisting of long-duration operations necessitating correction of unstable solutions of tin chloride, combined activation solution, solutions of chemical metallization, necessity of utilization of wastes of precious metals: palladium and rhodium. Furthermore, this process does not provide for a high reliability of interlayer junctions in printed boards, since the adherence of the metallization layer with the dielectric surface of the hole wall ensures only a 3-5-time resoldering of holes.